1. Field of the Invention
The present invention relates to an image forming apparatus. More particularly, the present invention relates to a tandem-type image forming apparatus.
2. Description of the Related Art
A tandem-type image forming apparatus includes a delivering belt for delivering a recording medium, a laser scanning unit (LSU) for scanning using a laser beam, a photosensitive drum, and a fixing unit. The photosensitive drum receives the laser beam to form a latent image, attaches and develops a toner to form a toner image, and transfers the toner image onto the transfer paper. The fixing unit then fixes the image formed onto the transfer paper. In such an image forming apparatus, a plurality of laser scanning units and a plurality of photosensitive drums are installed to develop an image using toners of different primary colors (such as cyan, magenta, yellow, and black). Color images are then formed by transferring and overlapping the different color images onto the transfer paper.
However, when the color image is formed using the conventional image forming apparatus, if the overlapping of the different color images is even slightly mismatched, an image characteristic is degraded. Therefore, an image forming apparatus having an adjusting unit for sensing and compensating for the mismatching of the image has been proposed. In the adjusting unit, toner patches consisting of respective colors are disposed on both edges of the delivering belt, and an optical sensor unit senses a position difference (that is, a color difference) of the toner patches. A correction amount such as a read timing or a magnification correction is calculated based on the color difference data, and thereby, the LSU or the photosensitive drum can be corrected.
The optical sensor unit for sensing the color difference of the toner patches includes a light source such as a light emitting diode (LED) or a semiconductor laser, a light receiving sensor for receiving a reflected or transmitted light and sensing a color difference of the toner patches, an optical lens and a driver circuit, as noted in Japanese Patent Nos. 63-300261 and 7-261628, the entire disclosures of which are incorporated herein by reference.
An image forming apparatus having an optical path changing unit has also been proposed. The optical path changing unit changes an optical path of a laser beam scanned from an LSU and guides the laser beam to a sensing position of the toner patch. A moving mirror is disposed on the optical path of the laser beam. When sensing the color difference of the toner patches, the optical path changing unit reflects the laser beam, which is emitted from the LSU, and guides it toward the sensing position using a fixed mirror as noted in Japanese Patent No. 2002-23450, the entire disclosure of which is incorporated herein by reference.
However, the first conventional image forming apparatus requires a number of expensive devices, such as the light source, the light receiving sensor, the optical lens and the driver circuit, resulting in an increased cost. Also, there is a limit to the optical lens when narrowing the laser beam of the LED. Since the LED has a short focal distance, it is susceptible to the distortion of the delivering belt. Therefore, if the LED is used as the light source, the sensing precision is degraded.
Further, in the second conventional image forming apparatus, the scan speed of the scanning laser beam is very fast as compared with the motion speed of the delivering belt. Therefore, the laser beam is only momentarily scanned on the sensing position, such that the sensing precision is degraded.
Accordingly, a need exists for a lower cost system and method for sensing a position difference to provide a quality image.